1. Field of the Invention
This invention relates to polyaldehydes, particularly polyoxymethylene polymers, containing photochemically degradable end groups and to articles and methods of relief imaging using the same.
2. Relation to the Prior Art
In U.S. Pat. No. 3,991,033 issued Nov. 9, 1976, to Sam, random copolymers containing polyoxymethylene chains with interspersed photosensitive units, wherein both ends are joined to oxygen, are described. The units have the formula ##STR4## wherein R.sup.1 and R.sup.2 are H or phenyl optionally substituted with up to 5 substituents of lower alkyl, preferably methyl, or --NO.sub.2, n is 0 or 1, R.sup.3 and R.sup.4 are H or lower alkyl, preferably H or methyl, with the provisos
(i) that at least one of R.sup.1 and R.sup.2 is a phenyl group having at least one orthonitro substituent, PA1 (ii) that when n is 1, R.sup.1 and R.sup.2 are o-nitrophenyl or H and o-nitrophenyl, said photosensitive linking units being present in an amount of from 0.001 to 0.05, preferably 0.002 to 0.01 unit per --CH.sub.2 O-- unit, said polymers having a number average molecular weight of from 1000 to 100,000, and preferably having an inherent viscosity of 0.7 to 1.5 measured in hexafluoroisopropanol (HFIP) in 0.5% concentration at 30.degree. C., which corresponds to a number average molecular weight for polymers of this type of about 15,000 to about 40,000.
The polymers of that invention are made by intercalation of preformed polyoxymethylene polymers with substituted dioxolanes, where n=0, and with substituted dioxanes, where n=1, of the formulae: ##STR5## In these formulae R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are as defined above. Dioxolanes are more readily obtained and are preferred.
Although the polymers of that invention may contain a photosensitive unit at the end of a polymer chain, they also contain photosensitive units randomly interspersed within the polymer chains because of their method of preparation by intercalation. In contrast with the prior art polymers, the polymers of the present invention contain photosensitive units only at the ends of the polymer chains because of their method of preparation.
Barzynski, et. al. U.S. Pat. Nos. 3,849,137 and 3,926,636, disclose light-curable compositions having a photosensitive polymer working layer containing o-nitrocarbinol ester groups.
Limburg, et. al. U.S. Pat. Nos. 3,915,704, 3,915,706, and 3,917,483, disclosed imaging compositions and methods based upon photoinduced degradation of a polyaldehyde catalyzed by an acid, generated photochemically from a potential or latent acid. The polyaldehydes do not contain photosensitive units.
Marsh, U.S. Pat. No. 3,923,514, discloses a process for the preparation of relief printing masters which uses a degradable polyaldehyde or polyketone and a photooxidant which is capable of abstracting an electron from an oxygen atom of the polymer. The polyaldehydes and polyketones do not contain photosensitive units.